This invention generally relates to plastic containers for retaining a commodity during a pasteurization or retort process. More specifically, this invention relates to plastic containers having a shoulder geometry that minimizes spherulitic crystallization below the finish during subsequent thermal processing of the container and/or a product within the container and a method for manufacturing a like container.
Numerous commodities previously supplied in glass containers are now being supplied in plastic, more specifically polyester and even more specifically polyethylene terephthalate (PET), containers. The manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities.
Manufacturers currently supply PET containers for various liquid commodities, such as beverages. Often these liquid products, such as juices and isotonics, are filled into the containers while the liquid product is at an elevated temperature, typically 68xc2x0 C.-96xc2x0 C. (155xc2x0 F.-205xc2x0 F.) and usually about 85xc2x0 C. (185xc2x0 F.). When packaged in this manner, the hot temperature of the liquid commodity is utilized to sterilize the container at the time of filling. This process and the containers designed to withstand it are respectively known as hot filling and hot fill or heat set containers. Hot filling works as an acceptable process with commodities having a high acid content. Non-high acid commodities, however, must be processed in a different manner and manufacturers and fillers also desire to supply PET containers for those commodities.
For non-high acid commodities, pasteurization and retort are the preferred sterilization methods. Pasteurization and retort both presents an enormous challenge for manufactures of PET containers in that heat set containers cannot withstand the temperature and time demands of pasteurization and retort.
Pasteurization and retort are both methods for cooking or sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above about 70xc2x0 C. (about 155xc2x0 F.), for a specified length of time (20-60 minutes). Retort differs from pasteurization in that higher temperatures are used, as is an application of pressure externally to the container. The pressure is necessary because a hot water bath is often used and the overpressure keeps the water, as well as liquid in the product, in liquid form above its boiling point temperature.
These processes present technical challenges for manufactures of PET containers, since new pasteurizable and retortable PET containers for these commodities will have to perform above and beyond the current capabilities of conventional heat set containers. Quite simply, the PET containers of the current techniques in the art cannot be produced in an economical manner such that they maintain their material integrity during the thermal processing of pasteurization and retort.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also know as the xe2x80x9ccrystallinityxe2x80x9d of the PET containter. Crystallinity is characterized as a volume fraction by the equation:   Crystallinity  =            ρ      -              ρ        a                            ρ        c            -              ρ        a            
where xcfx81 is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc).
The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse or radial axis. The combination promotes what is known as biaxial orientation in the container. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having about 20% crystallinity in the container""s sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of about 120xc2x0 C.-130xc2x0 C. (about 100xc2x0 F.-105xc2x0 F.), and holding the blown container for about 3 seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85xc2x0 C., currently use heat setting to produce PET bottles having a crystallinity range of 25-30%. Although heat set PET bottles perform adequately during hot fill processes, they are inadequate to withstand a pasteurization or retort process.
It should be noted that as the term is used herein, pasteurization is referring to pasteurization processes where pasteurization of the commodity occurs within the container. Also, a distinction needs to be made between pasteurization temperatures of the commodity internally of the container verses those temperatures applied exteriorly of the container to achieve the desired internal commodity temperature. Unless otherwise indicated, the pasteurization temperatures referenced herein will refer to the external temperatures applied to the container in order to achieve pasteurization of the contents within the container.
A further distinction needs to be made between the pasteurization of liquids and the pasteurization of solid commodities (herein those commodities containing a portion of solids, e.g. pickles), both of which generally require an internal pasteurization temperature of about 75xc2x0 C. (about 168xc2x0 F.). In the pasteurization of liquid commodities, pasteurization temperatures of about 68xc2x0 C.-79xc2x0 C. (about 155xc2x0 F.-175xc2x0 F.) are required to achieve the desired internal pasteurization temperature. Pasteurization of this variety is herein referred to as low temperature pasteurization. In the pasteurization of solid commodities, pasteurization temperatures of about 82xc2x0 C.-99xc2x0 C. (about 180xc2x0 F.-210xc2x0 F.) are required to achieve the desired internal pasteurization temperature, within generally the same amount of time. This is because of the lower thermal conductivity of the solid portions of the commodity. Pasteurization of this variety, where the pasteurization temperature is above 79xc2x0 C. (175xc2x0 F.) (the glass transition temperature of PET), is herein referred to as high temperature pasteurization.
For completeness, retort processes typically involves internal retort temperatures of 104xc2x0 C.-121xc2x0 C. (220xc2x0 F.-250xc2x0 F.) and external retort temperatures of 104xc2x0 C.-132xc2x0 C. (220xc2x0 F.-270xc2x0 F.). Unless specified otherwise, as used herein retort temperatures will be referring to external retort temperatures.
Since conventional heat set PET containers cannot withstand high temperature pasteurization and retort processing, the manufacturers of PET containers desire to produce a PET container that maintains aesthetic and material integrity during any subsequent high temperature pasteurization or retort of the contents in the PET container.
It is therefore an object of this invention to provide such a container that overcomes the problems and disadvantages of the conventional techniques in the art.
An object of this invention is therefore to provide a container capable of being subjected to high temperature pasteurization and retort while maintaining its aesthetic and material integrity.
Another object of this invention is to provide a container having high crystallinity levels (greater than 30%) in its sidewalls and clarity in the body of the container, from immediately below the support ring of the container to at least the base of the container.
Accordingly, this invention provides for a plastic container which maintains aesthetic and material integrity during any subsequent high temperature pasteurization or retort process, and during subsequent shipment and use.
Briefly, the plastic container of the invention includes a finish, a body portion and a base portion. The finish includes an opening defining the mouth of the container, a threaded portion (or other configuration) as a means to engage a closure, and a support ring that is used during handling, before, during and after manufacturing. The body portion includes a shoulder and a sidewall. The sidewall generally defines the greatest portion of the container""s diameter. The shoulder is that transition portion from just below the support ring to the sidewall. Both the shoulder and sidewall are provided with a high crystallinity, a crystallinity of greater than 30%. Relative to a generally transverse plane defined by the mouth of the container, the shoulder of a container according to the present invention flares outward from the finish at an angle of about 15xc2x0 or less, more preferably 10xc2x0 or less, and most preferably at about an angle of zero. By flaring outward at such a drastic or sharp angle, the PET material which defines the transition from unoriented to oriented material is restricted to the finish and moved out from the shoulder. As a result, during the thermal processing of the container which induces high crystallinity, as well as during the high temperatures and long duration of high temperature pasteurization and retort processes, the shoulder portion of the present container does not opacify and remains aesthetically acceptable.
Accordingly, in one aspect the present invention is a plastic container for retaining a commodity during a high temperature pasteurization or retort process. The container includes a finish defining an aperture for receiving the commodity, a body portion generally extending downward from the finish portion, and a base portion generally extending inward from the body portion and closing off the bottom of the container. The body portion itself has a crystallinity of greater than 30% and a shoulder portion flaring outward, from a generally transverse plane defined by the mouth of the container at an angle greater than 15xc2x0 or less.
In another aspect, the present invention is a method of forming a plastic container comprising the steps of: providing a preform having a finish within a mold; expanding the preform into conformity with a cavity of the mold to form a container having a body portion with a shoulder and a sidewall; stretching material forming the shoulder at an angle substantially restricting that portion of the material defining a transition from oriented to unoriented material to the finish and to induce orientation into the shoulder adjacent to the finish; thermally treating the container to crystallize the body portion; the orientation being induced in an amount sufficient to prevent opacifying of the material in the shoulder when the container is subjected to temperatures crystallizing the body portion to a crystallinity of greater than 30%.
In yet another aspect, the present invention is a method of forming a plastic container comprising: providing a preform having a finish with a support ring within a mold; expanding the preform into conformity with a cavity of the mold to form a container having a body portion with a shoulder and a sidewall; stretching material forming the shoulder at an angle relative to a transverse plane defined by the mouth of the container to induce orientation into the shoulder adjacent to the finish; heat treating the container to induce in the body portion crystallinity of at least 30%; the orientation being induced in the shoulder being sufficient to prevent opacifying of the material in the shoulder when the container is heat treated to induce in the shoulder crystallinity of at least 30%.
Further features and advantages of the invention will become apparent from the following discussion and accompanying drawings.